Cellulose ester lithographic printing process



P 1948. R. s. COLT 2,448,861

CELLULOSE ESTER LITHOGRAPHIC PRINTING PROCESS Filed Aug. 23, 1945 FIGJA.

HYDROLYZED sun/54c:

FIG. I B. 12 EXPOSED AREAS 11-1 FIG. lc.

,2 uusxrox'p HYDROLYZED LAYER J REMOVED WITH ZnCIz IO FIG ZA. IHYDROLYZED SURFACE co/vmuvma BK'HROMATE l 2ZLCELLULOSE ACETATE FIG. 2B-arms AREAS H1 [0'1 A FIG. 2C

. EXPOSED HYDROLYZED LAYER II I Rg'Ma/w WITH 500m urrocmolre RALPH$.COLT

Patented Sept. 7, 1948 2,440,001 cmumsa as'raa urnoammc PRINTING raocassRalph S. Colt. Rochester, N. Y., alllgnor to Eastman Kodak Company.Rochester,

poratlon of New Jersey Application sum 2:, ms. Serial No. 012,241

80mins.

This invention relates to the preparation of lithographic plates forprinting purposes.

Lithographic printing plates are generally made of metal having abichromate sensitized coating. Lithographic printing plates of celluloseester are known and have usually been made similarly to the metal platesby coating a cellulose ester sheet with a sensitive resist. Loening andMitson British Patent 565,752 describes a method for the preparation ofcellulose ester lithographic printing plates according to which a sheetof surface hydrolyzed cellulose ester is coated with a silver halide orbichrcmate sensitized colloid, an image formed in the sensitive layer,the unexposed parts of the stencil and hydrolyzed surface removed withzinc chloride and the remaining portions of the stencil then removedwith sodium hypochlorite. The unexposed parts of the cellulose acetateaccept fatty inks, and the exposed parts remain hydrophilic and repelthe ink.

It has also been proposed to impregnate the surface of a regeneratedcellulose sheet with bichromate and to depend upon the diil'erence inhydrophilic properties of the exposed and unexposed areas to render thesheet useable as a printing plate. A sumcient exposure to light of thesensitized surface makes it non-wettable so that differential inkingcanbe obtained in the exposed and unexposed areas. This method requiresa long exposure time and often gives inferior differentiation betweenink and waterreceptive areas.

It is an object of the present invention to provide a novel lithographicprinting plate. A further object is to provide a novel method of forminga design for printing purposes in a cellulose ester sheet. Other objectswill appear from-.the following description of my invention.

These objects are accomplished by more or less hydrolyzing a shallowregion, including the surface, of a cellulose ester sheet, which, whenN. Y., a ourlyzed surface layer which is hydrophilic and repelspenetrates entirely through the face layer the supporting layer mustprovide the ink-receptive areas of the final surface although it may ormay not be hydrolyzable. 1

According to one modification of my invention the hydrolyzed surfacelayer of the cellulose ester sheet is impregnated with a blchromate ,orchromate sensitizer such as potassium bichromate, or chromate, exposedunder a halftone or line positive and the unexposed hydrolyzed skinetched away with zinc chloride. The exposed hydrolyzed layer isinsoluble in zinc chloride al--.

though it remains wettable with water when the exposure is of relativelyshort duration, i. e., when it is a fraction of the exposure required torender the exposed regions water repellent. After washing off thezincchloride, the plate is .ready for printing. a

This method gives a more positive differentia- 1 ,tion between thewettable and non-wettable areas properly carried out, produces a toughlayer that may be regenerated cellulose, or a mixture thereof with someunchanged cellulose ester, impregnating the hydrolyzed surface layerwith bichromate or chromate, exposing it under a design and selectivelyremoving a portion of the hydrolyzed surface layer, either the exposedor the unexposed portion, with a substantiallyaqueous solution of aninorganic etching agent.

In the accompanying drawing, Figs. 1A, 1B and 1C are sectional viewsshowing one method of forming a cellulose ester printing plate accordingto my invention and Figs. 2A, 2B and 2C are sectional views of analternative method of forming a cellulose ester printing plateaccordingto my invention.

I use as the printing plate a sheet of cellulose ester, such ascellulose acetate, having a hydrothan does simple exposure withoutetching. A much shorter exposure can be used than is required to' makethe bichromated or chromated hydrolyzed surface unwettable.

My invention will now be described by reference to the following examplewhich is illustrative only.

Example 1 v A sheet of cellulose acetate propionate film base ishydrolyzed for five minutes in the following solution at roomtemperature:

Sodium hydroxide "ma.-. sov Ethyl alcohol cc 400 Water to 1 liter Thefilm is then washed with water and immersed in the following sensitizingbath for three minutes.

Potassium bichromate grams Ammonium hydroxide (28% solution) ..cc .25

Condensation product of an aldehyde with naphthalene sulfonic acid.-grams Water to .1 liter The sensitized coating is then drained withoutrinsing and dried, exposed under a positive transparency for 30 secondsto 5 minutes at 30 inchesfrom an are light and developedby swabbing for3 minutes with a Be. solution of zinc chloride.

This modification of my invention is illustrated H 3 in the accompanyingdrawing in which Fig. 1A is a sectional view of a cellulose acetatesheet l having a hydrolyzed surface layer H which is sensitized bybichromate or chromate. This material is exposed under a line positiveto form exposed areas I! in the hydrolyzed surface layer II, as shown inFig. 1B. The unexposed portions of the hydrolyzed surface layer II arethen removed in zinc chloride to leave an intaglio surface as shown inFig. 1C. The drawing does not correctly represent the thickness of thehydrolyzed and unhydrolyzed portions ofthe film but the thickness of thehydrolyzed portion is exaggerated for purpose of clarity ofillustration.

According to another modification of my invention, the bichromatedhydrolyzed cellulose acetate is softened, disintegrated, or dissolved,in a diflerent type of reagent after expose to light. The hydrolyzedsurface layer of the cellulose ester film is impregnated with abichromate or chromate sensitized, dried and exposed under a halftone orline image as before. In this case a negative halftone or line image ispreferably used and the exposed material is etched with a reagent suchassodium hypochlorite, sodium chlorite, or sulfuric acid whichdisintegrates the exposed hydrolyzed layer without affecting theunexposed areas.

This method afiords a better differentiation between wettabl andnon-wettable areas than does exposing a bichromated hydrolyzed areawithout etching. It can be used to convert a camera negative directlyinto a printing plate and does not depend upon the use of a wash-oilrelief emulsion, which must be developed in a tanning developer, or onany other type of-protective stencil. The use of a tanning developerresults in some loss of sharpness caused by a spreading of the tanningaction beyond the edges of the developed silver image, and requires moreprocessing operations.

This modification of my invention will be further illustrated byreference to the following example which is illustrative only.

Erample 2 A film having thereon a slow-speed, high-contrast silverhalide emulsion is exposed in a camera through a half-tone screen anddeveloped to a halftone negative. After fixing and washing. it isimmersed in the following hydrolyzing solution for five minutes at roomtemperature:

Sodium hydroxide -grams- Ethyl 3101101 on Water to 1 liter.

This treatment hydrolyzes the rear surface layer of the film whichalready carries a developed silver image on the opposite surface. Thefilm is then washed in running water for three minutes and thesensitizing bath, whose formula was disclosed in Example 1, is appliedto the rear surface with a cotton swab, a brush or by spraying. The filmis then drained without rinsing and dried, and exposed through theemulsion for 4 20% aqueous sulfuric acid or for a longer time withsodium chlorite solution produces the same effect. The last traces ofthe hydrolyzed cellulose ester softened by the sodium hypochlorite inthe exposed areas are removed with cotton or a piece of carpet and theplate is ready for printing.

It will be understood that the printing plate can be made from a sheetof cellulose ester not carrying a silver halide emulsion, by exposureunder an image on a separate support, but the method described above,provides for exposure in the camera and use of the same support forforming the printing plate, the negative image on the support beingremoved during processing of the printing plate.

This modification of my invention is illustrated in Figs. 2A, 2B and 20of the drawing. As shown in Fig. 2A a cellulose acetate support Inhaving a hydrolyzed surface layer ll containing a chromate or bichromatesensitizer is exposed under a line image to produce exposed areas i3 asshown in Fig. 23. On treatment of the film with sodium hypochlorite,sulfuric acid or exposed sodium chlorite areas are removed as shown atIt in Fig. 20 while leaving the unexposed hydrolyzed portions I l on theprinting surface.

In a further modification of my invention, surface-hydrolyzed celluloseester film is coated with a resist containing a water-insolublelight-sensitive compound such as dicinnimal acetone, or compositionscontaining cinnamal ketones and resins. After exposure through a screenpositive which decreases the solubility of the exposed parts, theunexposed parts are washed away in a solvent such as formamide ortriethanolamin-e. The film is then treated with zinc chloride whichremoves the formamide or triethanolamine and simultaneously etches thehydrolyzed surface layer of the cellulose ester where it is unprotectedby the resist. Upon removal of the resist a lithographic printing plateis formed which repels ink in the areas where the hydrolyzed areasremain.

It will be understood that in all modifications of my invention thehydrolyzed layer which remains on the surface of the final printingplate is hydrophilic and repels greasy inks. The unhydrolyzed materialis hydrophobic and carries the ink used for printing. Thus in themodification in which zinc chloride or a similar agent is used foretching the hydrolyzed layer, the unexposed areas which are etched withthe zinc chloride carry the printing ink. In the modification in whichsodium hypochlorite, sodium chlorite, sulfuricacid, or a similarmaterial is used, the exposed areas of the layer are removed and theseparts carry the printing ink.

The cellulose ester used in the formation of lithographic printingplates, according to my invention, must be one which is hydrolyzable andI prefer to use an ester of cellulose with a carboxylic acid or acidssuch as cellulose acetate. cellulose acetate propionate, celluloseacetate butyrate, etc. A layer of a cellulose ester such as cellulosenitrate which can be hydrolyzed only with difliculty may be covered witha layer of cellulose acetate or other hydrolyzable material and thislayer may then be hydrolyzed throughout. This method has thecharacteristic that the hydrolysis is more easily controlled in that itcannot penetrate further than through the layer of cellulose acetate andthe back of the sheet is not affected by the hydrolysis. Alternatively,a protective lacquer may be used on the reverse side of the celluloseacetate or other hydrolyzable sheet for the same purpose.

Sensitizers' used may consist of any watersoluble bichromate or chromatesuch as calcium bichromate or alkali metal salts such as sodiumbichromate, potassium bichromate or ammonium bichromate or chromate.alkaline bichromates or chromates appear to be preferable.

Where I refer to the formation of a design," it will be understood thatthis term is used in its broader sense to include line and halftoneimages, lettering,'maps, pictures and any other matter normally capableof representation on lithographic plates.

Various types of well-known greasy inks used in lithographic printingwhich are repelled by hydrophilic 'colloid's,- such as bodied linseedoil inks, may be used for the printing operation using the printingplates of my invention.

It will-be understood that the examples and modifications includedherein are illustrative only and that my invention is to be taken aslimited only by the. scope of the appended claims.

I claim:

1. The method of forming a design in a cellulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurface hydrolyzed cellulose organic acid ester sheet with ammoniumbichromate, exposing said surface to light under a design andselectively removing a portion of the exposed hydrolyzed surface with asolution selected from the class consisting of water-soluble chloritesand water-soluble hypochlorites.

2. The method of forming a design in a cellulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurfacerhydrolyzed cellulose ester sheet with a water-soluble salt ofchromic acid, exposing said surface to light under a design, andtreating said exposed surface with a solution of a water-solublehypochlorite to disintegrate the exposed portions of the hydrolyzed.surface layer.

3. The method of forming a design in a cel-' lulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurface hydrolyzed cellulose ester sheet with a water-soluble salt ofchromlc acid, exposing said surface under a design to less than about50% of the quantity of light required to make the exposed regionswater-repellent, and treating said exposed surface with a solution of awater-soluble hypochlorite to disintegrate the exposed portions of thehydrolyzed surface layer.

4. The method of forming a design in a cellulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurface hydrolyzed cellulose organic acid Neutral or slightly estersheet with ammonium bichromate, ex-

posing said surface to light under a design and selectivelydisintegrating a portion of the exposed hydrolyzed layer with a solutionof a watersoluble chlorite.

5. The method of forming a design in a cellulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurface hydrolyzed cellulose organic acid ester sheet with ammonium'bichromate, ex-

posing said surface to light under a design. se-

lectively disintegrating a portion of said exposed hydrolyzed surfacewith a solution of a water-soluble chlorite and selectively removing thedisintegrated portions by rubbing with a moist pad.

6. The method of forming a design in a cellulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurface-hydrolyzed cellulose ester of a carboxylic acid sheet withammonium chromate. exposing said surface to light under a design. andremoving only the exposed portions of said hydrolyzed surface bytreatment with a solution of sodium hypochlorite.

7. The method of forming a design in a cellulosic sheet for printingpurposes, which comprises impregnating a hydrolyzed surface layer of asurface hydrolyzed sheet of cellulose ester of a carboxylic acid withammonium bichromate, exposing said surface to light under a design andremoving only the exposed portions of the hydrolyzed surface bytreatment with a solution of sodium chlorite.

8. The method of forming a design in a eellulosic sheet for printingpurposes which comprises impregnating a hydrolyzed surface layer of asurface hydrolyzed sheet of cellulose ester of a carboxylic acid with analkali-metal salt 40 of a chromic acid, exposing said surface to lightunder a design, and removing only the exposed portions of the hydrolyzedsurface by treatment with a solution of sodium hypochlorite.

45 RALPH S. COLT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,943,388 Ostwald Jan. 16, 19342,020,901 Miller Nov. 12, 1935 56 2,043,905 Seymour June 9, 1936 FOREIGNPATENTS Number Country 7 Date 509,010 Germany Oct. 4, 1930 337,888 GreatBritain Nov. 13, 1930 60 505,752 Great Britain Nov. 27, 1044

